Distributed switch memory architecture

ABSTRACT

A distributed memory switch system for transmitting packets from source ports to destination ports, comprising: a plurality of ports including a source port and a destination port wherein a packet is transmitted from the source port to the destination port; a memory pool; and an interconnection stage coupled between the plurality of ports and the memory pool such that the interconnection stage permits a packet to be transmitted from the source port to the destination port via the memory pool.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the field of network switching andmore specifically to a switch architecture capable of transmittingpackets between ports in which a switch memory for temporarily storingpackets while forwarding decisions are made is configured as a pool thatis available to all ports in the switch.

2. Description of the Related Art

Network switches provide a solution to client congestion problems byswitching network traffic at high speeds between ports, rather thanhaving every user broadcast to every other user in the network. Networkswitches enable information (formatted into packets) to be switched fromone port to another port based upon the Ethernet addresses embedded inthe packets. Conventional network switches are formed with switch-nodesthat are interconnected to each other. Each switch-node typically has aphysical link to an interconnection matrix which switches data betweendifferent switch-nodes. The interconnection matrix between switch-nodestypically incorporate either a cross-bar or a shared-bus architecture.The cross-bar and shared-bus architectures permit the forwarding ofpackets from a switch-node to another switch-node once packet switchingdecisions are made. The packet switching decisions are performed byprocessing hardware incorporated within each switch-node. Additionally,a local static random access memory (SRAM) for temporarily storingingress and egress packets is incorporated within each of theconventional switch nodes.

Referring first to FIG. 1, there is seen a conventional switch system100 which is based on the cross bar architecture and which includesswitch-nodes 105, 110, 115 and 120. Switch-node 105 includes a localSRAM 105 a that is configured for storing ingress and egress packets andis organized according to a First-In/First-Out (FIFO) discipline inorder to prevent inversions in the packet order. A controller 105 bcontrols the FIFO queue of packets which are temporarily stored in thelocal SRAM 105 a. The processing logic 105 c performs switchingdecisions on the packets. A plurality of ports 105 d receive andtransmit the ingress and egress packets, respectively. Switch-node 105is further coupled to the cross-bar switch 125 for permitting packets tobe transmitted to other switch-nodes once switching decisions are madeby the processing logic 105 c. Similarly, each of the other switch-nodes(e.g., nodes 110, 115, or 120) includes a local SRAM, controller,processing logic, and ports, and are likewise coupled to the cross-barswitch 125. The cross-bar switch 125 is based on a meshedinterconnection matrix design and permits a packet from any port on aswitch-node to be forwarded to a port of any other switch-node onceswitching decisions are made for the packet.

Packets that must be switched between switch-nodes are required totravel via the cross-bar switch 125. A packet destined for a busy portin another switch-node can thus block other packets destined for othernon-busy ports, thereby resulting in a “head-of-line” blocking problem.For example, assume the packets 130 and 135 both originate from node 115whereby packet 130 is destined for node 110 while packet 135 is destinedfor node 120. Assume further that packet 130 is ahead of packet 135 inthe FIFO queue of SRAM 115 a of node 115. In this example, thedestination port of packet 130 in node 110 is busy and is unable toaccept incoming packets, while the destination port of packet 135 innode 120 is not busy. Thus, packet 130 is required to wait until thedestination port in node 110 is available to receive data. Transmissionof packet 135 is also blocked until packet 130 is transmitted, eventhough the destination port of packet 135 is ready. Thus, thehead-of-line blocking problem can lead to undesirable performance suchas packet transmission delay.

FIG. 2 illustrates a switch system 150 which incorporates the shared-busarchitecture and which includes switch-nodes 160, 165, 170, and 175. Ashared-bus 185 connects switch-nodes 160, 165, 170, and 175 together andis local to a PCB card. Each of the switch-nodes 160-175 includeselements performing similar functions as those in switch-nodes 105-120of FIG. 1. For example, switch-node 160 includes a local SRAM 160 a,FIFO controller 160 b, processing logic 160 c, ports 160 d andconnections to shared-bus 185 for permitting packets to be transmittedto other switch-nodes once switching decisions are made by theprocessing logic 160 c. Similarly, the other switch-nodes (e.g., nodes165, 170, or 175) each include a local SRAM, controller, processinglogic, ports and connections coupled to the shared bus 185. Theswitch-nodes 160-175 follow a standard arbitration scheme (e.g., timedivision multiplexing, round-robin arbitration, etc.) so that aswitch-node can access the shared bus 185 and transmit a packet via theshared-bus 185 to another switch-node.

A disadvantage of the shared-bus design in FIG. 2 is as follows. Byadding switch-nodes to the shared-bus 185, the load of the shared-bus isincreased. An increased load limits the frequency of operation of thenetwork switch 150, thereby limiting switching capacity. Additionally,due to the shared configuration of the bus 185, blocking effects mayoccur in the shared-bus switch system 150 of FIG. 2. One example of suchblocking effects is the head-of-line blocking problem which wasdiscussed above.

The cross-bar switch system 100 (FIG. 1) and the shared-bus switchsystem 150 (FIG. 2) also have the following disadvantages. As statedabove, switch-nodes 105-120 (FIG. 1) and switch-nodes 160-175 (FIG. 2)each include, respectively, a local SRAM for storing ingress and egresspackets before packets are transmitted to other switch-nodes. However,SRAM devices are expensive (as compared to dynamic random access memory(DRAM) devices). In the conventional switch systems 100 and 150 (FIG. 1and FIG. 2, respectively), SRAM devices of sufficient sizes can beimplemented, but this option leads to higher cost.

Alternatively, the sizes of the SRAM devices can be made smaller toreduce cost, but decreasing the memory sizes will limit the bandwidthcapacity of the switch system. A limited bandwidth capacity leads to alimited switching capability. Additionally, the conventional switchsystems 100 and 150 require additional hardware to implement theswitch-nodes in the network, thereby resulting in additionalimplementation costs.

One conventional approach is to use chassis-based designs to implementthe switch-nodes and the switch systems. However, chassis-based designsalso increase the overall cost of switch systems. In addition,chassis-based designs have poorer integration characteristics, sincethese designs require a given amount of logic to be implemented inmultiple cards. Additional logic is then needed to serve as an interfacebetween the multiple cards.

Accordingly, there is a need for a switch memory architecture whichovercomes the above-mentioned deficiencies of conventional switchsystems and which is less expensive to implement. The present inventionfulfills this need, among others.

SUMMARY OF THE INVENTION

An object of the invention is to prevent the occurrence of thehead-of-line blocking problem of the prior art.

Another object of the invention is to provide a switch system that isless expensive than conventional systems and that has a higher bandwidthand switching capability than conventional systems.

Another object of the invention is to provide a switch system that caneasily incorporate advances in DRAM technology (e.g., higher speed,higher bandwidth or higher capacity).

Another object of the invention is to provide a switch system whichpermits memory to be dynamically allocated between switch ports.

Another object of the invention is to provide a switch system capable ofhigher integration and lower implementation costs as compared toconventional systems.

Another object of the invention is to provide a switching logic whichcan be implemented with smaller PCBs, which lead to small form factorsystems.

Another object of the invention is to provide a switch system capable ofpermitting the achievement of wire speed switching for packets of anysize between 64 bytes to about 1518 bytes, and up to about 9 k bytes forjumbo packets.

Another object of the invention is to provide a switch system which canbe easily scaled by number of ports and/or amount of memory withoutlimiting the system's switching capacity.

The present invention fulfills these objects, among others. According toone aspect, an apparatus according to the invention broadly provides adistributed memory switch system for transmitting packets from sourceports to destination ports, comprising a plurality of ports including asource port and a destination port wherein a packet is transmitted fromthe source port to the destination port; a memory pool; and aninterconnection stage coupled between the plurality of ports and thememory pool such that the interconnection stage permits a packet to betransmitted from the source port to the destination port via the memorypool.

In the immediate foregoing distributed memory switch system, theinterconnection stage comprises a switch stage connected to theplurality of ports and a memory switch connected to the switch stage andto the memory pool. The switch stage and the memory switch can beimplemented by one or more ASICs.

The present invention further provides a switch system for switchingpackets between ports, comprising an interconnection stage configured totransmit packets between ports; and a memory pool coupled to theinterconnection stage for storing packets which are received from theports.

According to another aspect of the invention, the present inventionbroadly provides a method for transmitting packets from source ports todestination ports, comprising the steps of: detecting the arrival of apacket from a source port; determining the address locations in a memorypool for buffering the packet after the packet is received from thesource port of the packet; buffering the packet in the memory pool afterthe packet is received from the source port; retrieving the packet inthe memory pool; and transmitting the packet from the memory pool to thedestination port of the packet.

The list of objects and possible advantages and benefits above is notnecessarily exhaustive and further advantages and benefits will becomeapparent upon studying the detailed description of the inventionprovided hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a switch system based on the cross-bar architecture;

FIG. 2 illustrates a switch system based on the shared-bus architecture;

FIG. 3 illustrates a memory switch system according to a firstembodiment of the present invention;

FIGS. 4A to 4C illustrate alternative implementations of the memoryswitch system consistent with the principles of the present invention;

FIG. 5 further illustrates an example of a Port ASIC that can beincluded in the memory switch system according to the inventionillustrated in FIG. 3;

FIG. 6 is a flowchart illustrating the method implemented by a Port ASICsuch as that illustrated in FIG. 5 as a packet is received from a sourceport and buffered in the memory pool of FIG. 3;

FIG. 7 is a flowchart illustrating the method implemented by a Port ASICsuch as that illustrated in FIG. 5 as a packet is retrieved from thememory pool of FIG. 3 and is then transmitted to a destination port;

FIG. 8 illustrates another example of a Port ASIC that can be includedin the memory switch system according to the invention illustrated inFIG. 3;

FIG. 9 further illustrates an example of a Memory ASIC that can beincluded in the memory switch system according to the inventionillustrated in FIG. 3;

FIGS. 10A and 10B further illustrate examples of a memory pool that canbe included in the memory switch system according to the inventionillustrated in FIG. 3;

FIG. 11 illustrates an alternative implementation of a Memory ASIC and amemory pool consistent with the principles of the present invention; and

FIG. 12 is a memory switch system according to another embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring in detail now to the drawings wherein similar parts or stepsof the present invention are identified by like reference numerals,there is seen in FIG. 3 a schematic block diagram of a memory switchsystem 200 in accordance with a preferred embodiment of the presentinvention. The switch system can switch packets between nodes in a localarea network (LAN) or different network segments or different networksin a wide area network (WAN).

Switch system 200 is capable of switching packets between a plurality ofports, shown configured as four sets of ports 210 a-210 d, with each sethaving 1 to n ports. The ports 210-1 . . . 210-n can be implemented byport modules such as an 8×10/100 Mb port module (100 Base TX), a1-Gigabit port module, or a 4-port 100 Base FX module. The ports 210-1 .. . 210-n can each also include a WAN module such as a module capable ofT1/T3/E1/E3 operations in TDM, frame relay, or ATM formats.

A switch stage 215 is comprised of four Port ASICs 245 a-d and iscoupled to a memory switch 220 via an interconnect 217, while a memorypool 225 is coupled to memory switch 220. Each port ASIC 245 a-245 drespectively interfaces with ports 210 a-210 d and acts to transferpackets between the ports 210-1 . . . 210-n and memory pool 225 viamemory switch 220. Accordingly, packet 250 can be transmitted from anyone of the ports 210-1 . . . 210-n associated with a Port ASIC, viamemory pool 225, to another of the ports 210-1 . . . 210-n associatedwith the same or any of the other Port ASICs. The switch stage 215,interconnect 217 and memory pool 225 can therefore be collectivelyreferred to as an interconnect stage that interconnects the individualports with the common memory pool 225.

A switch engine 230 and a central processing unit (CPU) 235 communicatewith the Port ASICs in switch stage 215 so that a packet 250 can betransmitted from its source node to its destination node, as elaboratedupon more fully hereinafter. Table RAM 240 is coupled to switch engine230 and can be implemented by a conventional RAM which is available fromnumerous memory suppliers. Switch engine 230 maintains Table RAM 240 sothat it lists addresses corresponding to temporarily stored packets 250as will be described in more detail below.

CPU 235 can be implemented by, for example, the MC 68360 microprocessorfrom Motorola, Inc. of Schaumberg, Ill. CPU 235 sets up the initialconfiguration of switch system 200, as elaborated more fullyhereinafter. CPU 235 may also gather statistics and other managementinformation from the packet flows, run diagnostics, and report systemserrors, as is conventionally known.

Switch engine 230 performs the packet switching determination operationsfor forwarding packets 250 received from the ports 210-1 . . . 210-n. Anexample of a switch engine which can perform the above operations isdescribed in commonly assigned U.S. patent application No. 09/058,335,entitled “Method and Apparatus for Multiprotocol Switching and Routing”,filed on Apr. 10, 1998, which is fully incorporated herein by reference.An advantage of implementing the invention with the switch engine of theco-pending application is that processor overhead is minimized sincedecision-making tasks on packet switching are efficiently allocatedbetween the CPU 235, and the dedicated ASICs 230, 245 a-245 d, and 252a-252 d. However, the present invention is not limited to use with theswitch engine in the co-pending application; rather, other switchengines may be used.

As described above, switch stage 215 is implemented by four applicationspecific integrated circuit (ASIC) elements which are specifically shownin FIG. 3 as Port ASICs 245 a, 245 b, 245 c, and 245 d. Likewise, theMemory Switch 220 can be implemented as four ASICs which are shown asMemory ASICs 252 a, 252 b, 252 c, and 252 d. The Port ASICs 245 a-245 dare coupled to the Memory ASICs 252 a-252 d to form a 4×4interconnection stage 217, which can be implemented by, for example,sixteen fast serial connections (i.e. SerGig) connected and arranged asshown in FIG. 3.

In FIG. 3, the Port ASICs and the Memory ASICs are shown in separateblocks to assist in describing the functionality of the presentinvention. Based on the teachings of the invention, however, thoseskilled in the art will realize that many different implementations arepossible. For example, as shown in FIG. 4A, each of the separate PortASICs can communicate with respective switch ASICs having thefunctionality of switch engine 230. Alternatively, as shown in FIG. 4B,the functionality of the Port ASICs and switch engine can be combinedinto single respective ASICs that communicate with the CPU and with theMemory ASICs via the interconnection stage. As a further alternative, asshown in FIG. 4C, the functionality of the Port ASICs, switch engine andMemory ASICs can be implemented by a single VLSI device formed on asingle semiconductor substrate that directly communicates with thememory pool and the CPU.

As for the implementation shown in FIG. 3, the Port ASICs 245 a-245 deach concentrate a large number of low-bandwidth data streams receivedfrom the multiple ports 210-1 . . . 210-n (e.g. n=8, 16, 24, etc.), andconvert the low-bandwidth data streams into a low number (e.g. 4, 8, 12,etc.) of high-bandwidth data streams which are received by the MemoryASICs 252 a-252 d. Accordingly, the Memory ASICs 252 a-252 d arepreferably optimized to switch a smaller number of high-bandwidth datastreams. This optimization arises from the feature that a given MemoryASIC has a pin budget which determines the amount of data streams whichcan be concurrently received by the given Memory ASIC. Additionally, theabove optimization arises from the feature that the memory devices 227a-227 d (in the memory pool 225) can transfer data at a higher rate thandata is communicated via ports 210.

The efficiency achieved by optimizing the Memory ASICs 252 a-252 d toswitch a smaller number of high-bandwidth streams is further noted inthe following comparison. A standard 100 Mbit MII (Media IndependentInterface) requires twenty (20) pins to transfer 200 Mbits of data. Incontrast, in the configuration shown in FIG. 3, sixteen (16) pins ofMemory ASICs 252 a-252 d can each carry 960 Mbits of data, if theinterconnect between switch stage 215 and 220 is clocked at 60 MHz.

Memory pool 225 can be formed by a plurality of DRAM type devices 227 a,227 b, 227 c, and 227 d. One example of a DRAM device which can be usedto implement devices 227 in memory pool 225 is an 8-Mbit Rambus DRAM,which is available from various memory suppliers. According to apreferred embodiment of the invention, the total bandwidth of the memorypool 225 is set at a value that is at least (or greater than) the sumtotal of the bandwidths of the ports 210-1 . . . 210-n. This permits thefull bandwidth at the ports 210-1 . . . 210-n to be supported by thememory pool 225 at any given time. Additionally, by setting the memorypool 225 bandwidth at a value which is greater than the sum total of thebandwidths of the ports 210-1 . . . 210-n by a given margin, someinefficiencies are also accounted for as data flows through the switchsystem 200.

An example for determining peak bandwidth values for the memory pool 225is as follows. If 32×100 Mbit ports are used in the switch system 200(e.g., each switch node 205 includes 8×100 Mbit ports 210-1 to 210-8),then the sum total of the bandwidths is equal to about 6.4 Gbits/s (6.4Gbits/s=32×100 Mbits/s×2, wherein the factor of 2 accounts forfull-duplex traffic). Based on the 6.4 Gbits/s port bandwidths sumtotal, a memory pool 225 with a peak bandwidth of, for example, about 15Gbits/s should be implemented to provide an adequate margin againstinefficiencies.

As is known, Ethernet packets are carried in frames of between 64 and1518 bytes. MAC layer components (not shown) are used to convert theframes, serially transmitted through ports 210, into packets 250 havinga predetermined size of, for example, 64-bytes. Thus, a 128-byteEthernet frame is received by the corresponding Port ASIC in switchstage 215 in two (2) 64-byte packets 250. Further control signals areprovided by the MAC layer components to signal the start and end of aframe. It should be noted, however, that the present invention is notlimited to the above-described Ethernet example; rather the presentinvention is adaptable for use in other packet switching technologies aswell. Moreover, packet sizes other than 64 bytes may be implemented.

In FIG. 5 there is shown a Port ASIC 245 which can be an implementationof Port ASICs 245 a, 245 b, 245 c, and/or 245 d according to a preferredembodiment of the invention. It includes receive (RX) and transmit (TX)queues 22 and 24 associated with each port 210-1 . . . 210 n with whichPort ASIC 245 interfaces. Packet deconstruct module 26 and packetreconstruct module 28 are responsible for splitting up andreconstructing, respectively, packets that are transmitted between ports210 and memory switch 220. Switch interface 30 communicates with switchengine 230 and address table 20 stores addresses corresponding toregions in memory pool 225 associated with each respective port 210. CPU235 initializes the settings in address table 20. CPU 235 can divide thetotal memory capacity evenly between all the ports in the system, or itcan assign larger spaces, for example, for higher speed ports andsmaller spaces, for example, for lower speed ports. CPU 235 or otherinstrumentalities can also dynamically update the settings in accordancewith, for example, loads on certain ports.

In operation, a 64-byte packet 250 received at one of ports 210 by PortASIC 245 is distributed (or divided) into four 16-byte packet portions(or other size portion, depending on the size of the packet 250 and thenumber of Memory ASICs) 250 a, 250 b, 250 c, and 250 d and stored inmemory pool 225 via memory switch 220, as will be explained now in moredetail with reference to FIG. 6. Switch interface 30 detects the arrivalof a given packet 250 in one of the RX queues 22-1 . . . 22-n associatedwith ports 210-1 . . . 210-n (step S10). Switch interface 30 determines,from address table 20, the range of memory addresses within memory pool225 for storing the given packet 250 based on from which of the ports210-1 . . . 2210-n the given packet 250 arrived (step S20). Switchinterface 30 immediately forwards a copy of the packet 250 (assuming itis the first packet in the frame, as determined in step S30, for exampleby determining whether it is the first packet received after a start offrame signal) to switch engine as) 230 (step S40). Alternatively, theswitch engine 230 can independently receive a copy of the first packetin the frame by other instrumentalities. The first 64 bytes of anEthernet frame will include the frame header information that the switchengine 230 will use to determine how to forward the frame. Switchinterface 30 also forwards a message to switch engine 230 that includesthe memory pool 225 addresses at which the given packet 250 will bestored (step S50). Switch engine 230 will store this address in TableRAM 240, along with an identifier for the frame and then begin itspacket forwarding determination operations.

Switch interface 30 forms a command 251 for relaying along with thesplit packet portions and sends it to memory interface 32 (step S60).The command includes the address at which the portion is to be stored inmemory, as well as an indicator indicating that a memory write operationis to be performed. Packet deconstructor 26 splits the given packet 250into the packet portions 250 a-250 d (step S70). When memory interface32 receives the packet portions and command, it transfers the packetportions and appended command to the appropriate Memory ASICs 252 a-252d so that the packet portions are stored in the proper memory pool 225addresses (step S80). For example, a 64-byte packet 250 is formed byByte[0] . . . Byte[63]. Packet deconstructor 26 splits the packet, andmemory interface 32 sends the portions, so that Memory ASIC 252 areceives the packet portion 250 a, i.e., Byte[0] . . . Byte[15], whileMemory ASIC 252 b receives the packet portion 250 b, i.e., Byte[16] . .. Byte[31], Memory ASIC 252 c receives the packet portion 252 c, i.e.,Byte[32] . . . Byte[47], and Memory ASIC 252 d receives the packetportion 252 d, i.e., Byte[48] . . . Byte[63]. By distributing the64-byte packet 250 into four equal sized packet portions 250 a-250 d, itis ensured that the bandwidth load is always equally distributed acrossthe four memory devices 227 a-227 d (see FIG. 3) of the memory pool 225(see FIG. 3) and that overloading does not occur in any of theindividual Memory ASICs 252 a-252 d. Although an equal loading amongmemory devices is preferred, it is not necessary and otherimplementations are possible.

It should be noted that frames may have an arbitrary size that result indata not fully consuming a 64-byte packet. For example, an 80-byte framewill consume one 64-byte packet and 16 bytes of a subsequent packet.Processing can be further performed to ensure that these odd portionsare equally loaded among memory devices. For example, if an 80-byteframe is received from port 1, the Port ASIC will cause the leftover16-byte portion to be stored in memory device 227 a via Memory ASIC 252a. On the other hand, if the 80-byte frame is received from port 2, thePort ASIC will cause the leftover 16-byte portion to be stored in memorydevice 227 b via Memory ASIC 252 b. It should be apparent that otherload balancing techniques are possible, such as, for example, by makinga determination based on addresses.

A circular buffer structure is maintained for each port, and the packetsare preferably placed “back-to-back”. For example, if a packet receivedat a given port is stored at addresses 0 . . . N in the memory pool 225,switch interface 30 will keep track so that the initial portion of anext received packet for the given port is stored at address N+1. Thiswill continue until the entire address range for the port has beenfilled, in which case switch interface 30 will reset the startingaddress for stored packets to the beginning of the address range for theport.

If the packet that was stored was not the last packet in the frame (asdetermined in step S90, e.g. no signal has been received signaling theend of the frame), control will return to step S10 and the Port ASICwill continue to receive and store packets for the frame in memory pool225. When the last packet is received, switch interface 30 will notifyswitch engine 230 to that effect (S100) and the Port ASIC will awaitfurther frames.

In operation, a 64-byte packet 250 transmitted via one of ports 210 byPort ASIC 245 is reconstructed from four 16-byte packet portions(chunks) 250 a, 250 b, 250 c, and 250 d that were retrieved from memorypool 225 via memory switch 220, as will be explained now in more detailwith reference to FIG. 7. When switch engine 230 determines how a framemust be forwarded, it looks up the address for the packet in Table RAM245 and sends a message to switch interface 30 of the Port ASIC 245associated with the destination port of the frame. The message includes,for example, the address in memory where the frame is stored, the sizeof the frame, and the destination port number. Switch interface 30 ofthe associated Port ASIC 245 receives the message (S200) and creates acommand 251 to be sent to the memory switch (S210). The command includesthe address in memory where the packet is stored, as well as anindication that a memory read operation is to be performed.

The packet portions 250 a-250 d are received by memory interface 32 inresponse to the command (step S220). Memory interface 32 sends them topacket reconstructor 28 which reconstructs the received packet portions250 a-250 d into the given packet 250 (step S230). When the entire 64bytes have been received, packet reconstructor 28 relays the packet 250to the TX queue 24 associated with the packet's destination port (ofports 210-1 . . . 210-n) (step S240). If this is not the last packet inthe frame (determined in step S250), switch interface 30 determines thememory pool address of the next packet in the frame (step S260), andcontrol returns to step S210 for retrieving the next packet from memorypool 225. Otherwise, control returns to step S200 for waiting for thenext message from switch engine 230.

FIG. 8 shows an alternative embodiment of the Port ASIC, generally shownas 245′, according to the invention. The packets 250 a′, 250 b′, 250 c′,and 250 d′ are received at time t1, t2, t3, and t4, respectively. Eachof the packets is sized at, for example, 64 bytes. The Port ASIC thenoutputs the packets 250 a′, 250 b′, 250 c′ and 250 d′ at time t1′, t2′,t3′ and t4′, respectively, which are also 64 bytes, not 16 bytes as inthe previous example. Accordingly, load-balancing of stored packetsbetween areas of the memory pool 225 is achieved but in a differentmanner than described above.

FIG. 9 shows a Memory ASIC 252 which can be an implementation of theMemory ASICs 252 a, 252 b, 252 c, and/or 252 d (FIG. 3), and whichreceives the 16 byte packet portions 250 a-250 d according to apreferred embodiment of the invention. A plurality of full duplexchannels 300, 305, 310, and 315 couples the Memory ASIC 252 to themultiple ports via switch stage 215. Each of the channels 300-315enables the bi-directional transmission of a 16-byte packet portion(e.g., packet portion 250 a), or 64-byte packet portion in the exampleof FIG. 8, or other size portion depending on implementation.

In the example of FIG. 9, the Memory ASIC 252 is shown as having onlyfour (4) channels. However, an N number of channels can be implementedfor the Memory ASIC 252, with the number of channels being limited bydie size, pin count, and other constraints based on whether the ASIC isto be implemented on a single integrated circuit chip or multipleintegrated circuit chips.

The channel 300 includes the receive (RX) path 320, the transmit (TX)path 322, the RXDATA FIFO 324, the Command FIFO 326, and the TXDATA FIFO328, with the operation of these elements being elaborated upon morefully hereinafter. Similarly, the channel 305 includes the RX path 330,the TX path 332, the RXDATA FIFO 334, the Command FIFO 336, and theTXDATA FIFO 338. The channel 310 includes the RX path 340, the TX path342, the RXDATA FIFO 344, the Command FIFO 346, and the TXDATA FIFO 348,while the channel 315 includes the RX path 350, the TX path 352, theRXDATA FIFO 354, the Command FIFO 356, and the TXDATA FIFO 358.

The RX paths (e.g., RX path 320) are used for transmitting the packetportions (e.g., packet portion 250 a) from switch stage 215 to thememory pool 225. The TX paths (e.g., TX path 322) are used fortransmitting the packets portions from the memory pool 225 to the switchstage 215. The Memory ASIC 252 further includes the arbitration hardware360 and the Memory Controller 365. The arbitration hardware 360 isimplemented based on, for example, a standard round robin scheme whichgives fair access to each of the channels 300-315. The memory controller365 can be implemented by a standard memory data controller, and theimplementation is dependent upon the type of memory technology (e.g.,SDRAMs, Rambus DRAMs, Dual Data Rate DRAMs, etc.) used in memory pool225.

In operation, assume that command 251 a and/or packet portion 250 a areto be communicated with Memory ASIC 252. The Command FIFOs 326, 336, 346and/or 356 process the receive commands and transfer commands asindicated by the given command 251 a (e.g. write=receive,read=transfer). The received command 251 a is stored in the Command FIFOupon receipt. The receive commands serve to permit the data packetportions 250 a-250 d to be written into the memory pool 225, while thetransfer commands serve to permit the data packet portions 250 a-250 dto be read from the memory pool 225. The RXDATA FIFOs (e.g., FIFO 324)buffer the RXDATA (which is, e.g., a received data packet portion 250 ato be written into the memory pool 225), while the TXDATA FIFOs (e.g.,FIFO 328) buffer the TXDATA (which is, e.g., a data packet portion 250 awhich has been read from the memory pool 225).

The channel 300 (or any of the other channels) is “ready” to transferdata to or from the memory pool 225 once a “full command” 251 and itsassociated data 250, if any, are presented in the Command FIFO 326. Thearbitration hardware 365 arbitrates between all channels which are readyto transfer data and determines which of the ready channels will accessand transfer data to and from the memory pool 220 at a given time. Asstated above, a standard round-robin arbitration scheme, for example, isused, to implement the arbitration hardware 360. The channel which ispermitted to access the memory pool 225 will read from or write to thememory pool 225. For a receive command, the packet portion 250 a (andthe packet portions 250 b-250 d) is written from RXFIFO 324 and bufferedin memory pool 225 addresses indicated by the receive command. For atransfer command, the packet portion 250 a (and the packet portions 250b-250 d) is read from memory pool 225 addresses indicated by thetransfer command and stored in TXFIFO 328. The packet portion 250 a isthen output to the Port ASIC 245 corresponding to the channel.

As mentioned above, memory pool 225 can be implemented in a number ofways. The use of DRAM devices to implement the memory pool 225 leads tothe following possible advantages. First, the DRAM devices in the memorypool 225 as described above leads to lower system cost, but also allowshigher bandwidth capability than in conventional systems which do notuse the memory pool configuration. In contrast, prior art switch systemsrequire a local SRAM to be implemented in each switch node, as shown inFIGS. 1 and 2. Additionally, since SRAM devices are more expensive thanDRAM devices, the use of SRAM devices can increase system cost by asmuch as about three to four times as compared to the present invention.

Another advantage made possible by the invention is the higher memorycapacities provided by the DRAM devices which implement the memory pool225. Higher memory capacities lead to a higher switching capability forthe memory switch in accordance with the present invention. Further, theinvention can easily incorporate advances in DRAM technology (e.g.,higher speed, higher bandwidth or higher capacity). By changing thenumber of DRAM ports in the Memory ASICs 252 a-252 d and/or by changingthe type of memory technology which is implemented in the memory pool225, the switching capacity of the present invention can be increased.Examples of memory pool 225 peak bandwidth values are shown below forspecific memory implementations:

Example 1: A peak bandwidth of 3.84 Gbits/s is achieved by configuringone 64-bit SDRAM port at 60 MHz per Memory ASIC 252 a, 252 b, 252 c or252 d.

Example 2: A peak bandwidth of 16 Gbits/s is achieved by configuring two64-bit SDRAM ports at 125 MHz per Memory ASIC.

Example 3: A peak bandwidth of 19:2 Gbits/s is achieved by configuringfour Rambus DRAM ports at 600 MHz per Memory ASIC.

As memory technology advances, the advances can be incorporated in thememory switch system of the invention by appropriately configuring theMemory ASICs 252 a-252 d and the memory pool 225 as discussed above.Other parts of the switch system 200 need not be affected. Additionally,as alluded to above, the ports 210-1 . . . 210-n can share the use ofthe memory pool 225, and the memory pool 225 is available to be sharedunequally by all ports 210-1 . . . 210-n. Thus, each port 210-1 . . .210-n is not subject to a “fixed-size” limitation. In contrast, underthe more restrictive “fixed-size” design, ports of a given node can onlyshare memory addresses which are available for that given node.

Moreover, it is possible to utilize the memory banks of DRAM devices sothat, for example, a packet chunk is being received or stored in onememory bank of a given DRAM device, while another packet chunk is beingretrieved from the another memory bank of the same given DRAM device orfrom another DRAM device in the memory pool 225. This full duplexoperation capability thereby permits the system to achieve a higherbandwidth capability. In other words, the operations on different bankscan overlap. While a given packet chunk is being read from one memorybank, other memory banks can be setting up new pages for the next packetchunk transfer. In contrast, if packet traffic is concentrated in onlyone memory bank, the bandwidth of a particular system will be morelimited.

Commercially available memory devices are oriented towards PCapplications whereby sequential data is transferred for long burst, e.g.64 bytes or 128 bytes per transfer. Therefore, the page mode of DRAMscan be used efficiently when implementing the commercially availablememory devices for PC applications. In the present invention, however,in which each 16-byte chunk is transferred to or from a different page,page mode is not as advantageous and the following modifications areessential. First, for a 64-bit wide DRAM implementation, a burst-size of“2” is used (i.e., in two (2) cycles, 16-bytes will be transferred tothe DRAM). Second, the use of memory banks is maximized. Thus, data isspread across all available banks in the high bandwidth memory pool 225.By spreading data across available banks, the overlapping of memoryoperations is allowed, thereby permitting a greater bandwidth to beachieved. A commercially available memory typically has about two (2)memory banks, while Dual Data Rate DRAMs can have up to about eight (8)memory banks. Third, additional pins are added to each of the DRAMs foruse in DRAM signaling functions (such as “COMMAND” signals to the DRAMs.This permits a greater overlap to occur between COMMAND signals and DATAsignals. FIG. 10A illustrates an example of memory pool 225 wherein eachMemory ASIC interfaces with a memory device 227 having two memory banks.FIG. 10B illustrates an example of memory pool 225′ wherein each MemoryASIC interfaces with a memory device 227′ having four memory banks,which yields further overlapping of memory functions, and thus, greaterbandwidth over the implementation in FIG. 10A.

It should be understood that other implementations of Memory ASICs 252and memory pool 225 are possible while remaining within the principlesof the present invention. For example, as shown in FIG. 11, theinterface logic and FIFO functionality of the Memory ASIC 252 shown inFIG. 8, as well as the memory (e.g. four to sixteen banks) of memorypool 225 can be implemented together in a single semiconductor devicesuch as an ASIC.

Further advantages are obtained by utilizing Rambus memories toimplement the memory pool. Some of the advantages of using Rambus DRAMsto implement the DRAM devices in the memory pool 225 are as follows. ARambus DRAM (concurrrent type) typically operates at about 600 MHz with8 bits for data and a 31-pin interface. The peak per-pin-bandwidth istherefore (600 MHz*8 bits/31 pins) or 154 Mbits/s. In contrast, thebandwidth of conventional SDRAMs is limited by the SDRAM pin interface.The fastest commercially available SDRAM operates at about 143 MHz. Fora 64-bit implementation, about 80 pins are required. Thus, the peakper-pin bandwidth of an SDRAM is therefore (125 MHz*64 bits/80 pins) or100 Mbits/s. It should be noted that SDRAMs provide parallel synchronousbuses for data transfers. Due to pinout constraints and costconstraints, the largest bus that exists on a single SDRAM chip is32-bits in size. Multiple buses can be cascaded to form widerinterfaces. However, since the ASIC pinout costs increase significantlywith wider interfaces, practical limits today are buses of 64-bits, asfactored in the above calculation.

Currently available Rambus DRAMs provide a peak bandwidth of 4.8 Gbits/sand consume 31 pins. Thus currently available Rambus DRAMs average about154 Mbits per pin, (154 Mbits per pin=4.8 Gbits/s divided by 31 pins).About 4 Rambus DRAMs can interface with an ASIC. For example, in FIG.10B, each ASIC can interface with 4 Rambus DRAMs rather than a singledevice having four banks. As stated above, each Rambus DRAM provides apeak bandwidth of 4.8 Gbits/s. Therefore, each ASIC 252 a-252 d willhave peak bandwidth of about 19.2 Gbits/s or (4.8 Gbits/s * 4).Effective bandwidth in network applications ranges from about 60% toabout 75% of the peak bandwidth. Thus, the effective bandwidth of eachASIC 252 a-252 d will be about 12 Gbits/s or (19.2 Gbits/s * 60%). Thus,for a 96 Gbits/s system, about 8 ASICs are required to be implemented(96 Gbits/s=12 Gbits/s * 8 ASICs).

FIG. 12 shows another embodiment of the memory switch system, generallyshown as 600, according to the invention wherein a 6×4 interconnectionstage couples the switch nodes 205 to the memory pool 225. A Switch ASICstage 215′ comprises six Switch ASICs 245 a-245 f and is coupled betweenthe switch nodes 205 and the Memory Switch 220. This embodiment enablesthe above mentioned advantages and further achieves a higher bandwidthand switching capacity since the switch system 600 permits more nodes210-1 . . . 210-n to be added. It should be apparent that otherimplementations are possible, such as 8×4, 4×6, 4×8 and othervariations.

It should be noted that the memory switch system 200 and/or 600 can beimplemented by use of VLSI devices to connect the ports 210-1 . . .210-n together and to the memory pool 225. The interconnects arechip-to-chip based interconnections wherein chip-to-chip refers to adirect connection between two given ASICs (as opposed to having, forexample, a buffer between the ASICS). The chip-to-chip basedinterconnections enable higher integration and lower implementationcosts as compared to conventional approaches. In addition, since theswitching logic of the invention is highly integrated in the ASICs, theinvention can be implemented with smaller PCBs which lead to small formfactor systems.

It should be further noted that the high system level architecture andthe Switch ASICs and Memory ASICs implementations according to theinvention also permit wire speed switching for packet sizes betweenspecified bandwidth ranges, e.g., about 64 bytes to about 1518 bytes. Incontrast, conventional switch systems have to operate at predetermined“sweet spots” (e.g., 64 bytes etc.) in order for the conventional switchsystem to function correctly. In addition, a conventional switch systemis pre-designed to function at a predetermined sweet spot, and is unableto perform wire speed switching outside those sweet spots.

While the invention has been described in connection with what ispresently considered to be the preferred embodiments, it is understoodthat the invention is not limited to the disclosed embodiments. Forexample, each of the features described above can be used singly or incombination, as set forth below in the claims, without other featuresdescribed above which are patentably significant by themselves.Accordingly, the present invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A distributed memory switch system, comprising: aplurality of ports including a source port and a destination port,wherein a packet is transmitted from the source port to the destinationport; a dynamic random access memory (DRAM) memory pool; and aninterconnection stage coupled between the plurality of ports and theDRAM memory pool such that the interconnection stage permits the packetto be sent from the source port to the destination port via the DRAMmemory pool, the interconnection stage further splitting the packetreceived from the source port into a plurality of equal-sized packetportions for storage in the DRAM memory pool, wherein theinterconnection stage comprises a switch stage connected to theplurality of ports, and a memory switch connected to the switch stageand to the DRAM memory pool.
 2. The distributed memory switch system ofclaim 1 wherein the memory pool is comprised of memory banks forrespectively storing the packet portions.
 3. The distributed memoryswitch system of claim 1 wherein the switch stage comprises a first setof ASICs connected to the plurality of ports.
 4. The distributed memoryswitch system of claim 1 wherein the switch stage comprises at least oneASIC connected to the plurality of ports.
 5. The distributed memoryswitch system of claim 1 wherein the switch stage comprises at leastfour ASICs connected to the plurality of ports.
 6. The distributedmemory switch system of claim 1 wherein the switch stage determinesaddresses in the memory pool for storing the packet received from thesource port.
 7. The distributed memory switch system of claim 1 whereinthe interconnection stage reconstructs the packet portions retrievedfrom the memory pool into the packet to be sent to the destination port.8. The distributed memory switch system of claim 1 wherein the switchstage forms command signals which are associated with the packetreceived from the source port.
 9. The distributed memory switch systemof claim 1 wherein the memory switch comprises a second set of ASICsconnected to the switch stage and to the memory pool.
 10. Thedistributed memory switch system of claim 1 wherein the memory switchcomprises at least four ASICs connected to the switch stage and to thememory pool.
 11. The distributed memory switch system of claim 1 furthercomprising: a switch engine coupled to the interconnection stage formanaging the flow of packets between source ports and destination ports.12. The distributed memory switch system of claim 11 further comprising:a table RAM coupled to the switch engine.
 13. A switch system forswitching packets between ports, comprising: an interconnection stageconfigured to transmit the packets between the ports; and a dynamicrandom access memory (DRAM) memory pool coupled to the interconnectionstage for storing the packets which are received from the ports, whereinthe interconnection stage splits certain of the packets received fromthe ports into a plurality of equal-sized packet portions for storage inthe DRAM memory pool, the interconnection stage comprises a switch stageconnected to the ports and a memory switch connected to the switch stageand to the memory pool.
 14. The switch system of claim 13 wherein theswitch stage comprises a first set of integrated circuits connected tothe ports.
 15. The switch system of claim 13 wherein the switch stagecomprises at least one integrated circuit connected to the ports. 16.The switch system of claim 13 wherein the switch stage comprises atleast four integrated circuits connected to the ports.
 17. The memoryswitch system of claim 13 wherein the switch stage determines addressesin the memory pool for storing the packets received from the ports. 18.The switch system of claim 13 wherein the interconnection stagereconstructs the packet portions retrieved from the memory pool into thecertain packet associated with the packet portions.
 19. The switchsystem of claim 13 wherein the switch stage forms command signals whichare associated with certain of the packets received from the ports. 20.The switch system of claim 13 wherein the memory switch comprises asecond set of integrated circuits connected to the switch stage and tothe memory pool.
 21. The switch system of claim 13 wherein the memoryswitch comprises at least four integrated circuits connected to theswitch stage and to the memory pool.
 22. The switch system of claim 13further comprising: a switch engine coupled to the interconnection stagefor managing the flow of the packets between the ports.
 23. The switchsystem of claim 22 further comprising: a table RAM coupled to the switchengine.
 24. The switch system of claim 13 wherein the memory pool iscomprised of memory banks for respectively storing the packet portions.